This invention relates to devices that have optical interconnects, such as digital electro-optic switches and multiplexers. More particularly, this invention utilizes a hybrid waveguide structure referred to herein as a xe2x80x9cbuttonxe2x80x9d in an optical interconnect. New processes are also presented for fabricating circuits that incorporate these buttons.
Optical devices such as optical waveguides and switches are used in communications and data transfer equipment to transfer information from one location to another and to switch the information to a desired output. The information is in the form of a continuous or a pulsing optical signal.
These optical devices contain a core or cores made of a material that transmits light of the desired wavelength and cladding that abuts at least one side of a core. Optical waveguides are used to carry optical signals from one location to another. Multiple cores are used to form e.g. switches to switch an optical signal to a desired output core, filters to filter one or more optical signals of a particular wavelength, or multiplexers to combine or separate optical signals of different wavelengths. Optical cores can be linear, but often optical cores must curve in order to direct a signal from one location to another within the confines of a small space.
One major objective of electro-optic device research is to reduce the size of components. There are two benefits from reducing the size of components: (1) devices such as waveguides and electro-optic switches are shorter and/or smaller, allowing more components to be placed within an integrated device; and (2) signals are transmitted between components more quickly, which increases the speed at which data is transferred.
Currently, if the direction of an optical signal is to be changed 90xc2x0, the core must be fabricated to have a radius of approximately 10 mm to avoid losing much of the optical signal to the cladding in the curved section. Consequently, every 90xc2x0 turn that is incorporated along the length of a device adds at least 10 mm to the length or width of the device.
Another objective of electro-optic device research is to provide components that can be manufactured such that their switching characteristics are more consistent, so that a switch fabricated today performs essentially the same as a switch fabricated a month or year from today. Many switches have switching characteristics that are extremely sensitive to the voltage of the signal used to switch the optical signal from one output core to another or to distribute the optical signal among multiple cores. These switches are quite sensitive to manufacturing variances, and significant variations occur from one batch to the next of these switches or even within a batch of these switches.
An interferometric modulator as illustrated in FIG. 1 is a modulator whose performance is extremely sensitive to the voltage used to modulate the optical signal. This type of switch can be fabricated by diffusing a metal such as titanium into an electro-optic crystal such as LiNbO3 to form the cores. The titanium-diffused portion of the crystal (which is also electro-optic) has a higher refractive index than the virgin portion of the crystal, and consequently, the titanium-diffused portion acts as cores which carry an optical signal.
The interferometric modulator 100 as illustrated in FIG. 1 uses multiple cores to modify an input optical signal. The input optical signal is split between two input cores 110 and 120, and the two input cores separate from one another a sufficient distance that the cores do not evanescently couple. The optical signal in core 110 travels through that core unmodified. The second core 120 has a set of electrodes 130 fabricated above it, so that an electric field can be applied to the electro-optic material in that core. The optical signal in the second core can be unmodified as it travels through the core, or the optical signal can have its phase shifted in response to the electric field created by electrodes above and on either side of the core. The two cores subsequently recombine to form one core, where the optical signals add to one another. If the optical signals from each core are in phase in the section where the cores recombine to form one core, the signals add to form an optical signal having the same wavelength and phase. If the optical signals are out of phase, the optical signal that is output depends on how much the phase of the signal was shifted as it traveled through core 120.
The interferometric modulator of FIG. 1 can be very difficult to fabricate consistently. The amount of titanium diffused into the crystal is highly dependent on processing conditions, and the minor variations in processing conditions that occur during normal manufacturing processes cause an interferometric modulator produced in one batch to function very differently from an interferometric modulator made in another batch of switches when an identical electric field is applied to both switches.
It is an object of this invention to provide hybrid waveguide structures such as optical waveguides that have improved properties such as greater isolation, tight turning radii, or different propagation characteristics. It is another object of this invention to provide hybrid waveguide structures such as electro-optic switches that have less variance in their intended use because of the switch design and/or because of the process by which the switches are manufactured.
The invention provides a hybrid waveguide structure comprised of at least one core and cladding. At least a portion of a core and/or a section of its surrounding cladding has optical properties that differ from the optical properties of a neighboring core or portion of the same core or cladding area, respectively. Thus, in a hybrid waveguide structure, a core may have a short section along the length of the core that has a refractive index which differs from the refractive index of other sections along the length of the core. Additionally or alternatively, the hybrid waveguide structure has a core in which its refractive index differs from the refractive index of another evanescently-coupled core, and/or the cladding near a core may have a section that has a refractive index which differs from the remaining cladding around the core. The hybrid portion of the hybrid waveguide structure is referred to as a xe2x80x9cbuttonxe2x80x9d herein.
The invention also provides a hybrid electro-optic structure which has a portion of a core or a region of cladding made of an electro-optic material whose refractive index can differ from the refractive index of a neighboring portion of the same core or region of cladding, respectively. The refractive index of the electro-optic material can differ from the refractive index of its neighboring material in the presence of an applied electric field, or the refractive index of the electro-optic material can differ from the refractive index of its neighboring material in the absence of an applied electric field.
The invention also provides an integrated device having a hybrid waveguide structure and/or a hybrid electro-optic structure as described above.
In one embodiment, the invention provides a hybrid waveguide structure which in cross-section (as illustrated in FIG. 2) comprises three sections, a lower section 210, a middle section 220, and an upper section 230. Each section has a first, second, and third region when the structure has at least one core, and each section has a fourth and fifth region when the structure has at least two cores that are evanescently coupled. For a single-core structure, the regions are each formed of a material having a refractive index such that the second region of the middle section (222) is a core, and the first and third regions of the middle section are cladding under light-transmitting conditions. For a structure having two or more evanescently-coupled cores, the regions are each formed of a material having a refractive index sufficient that the second and fourth regions of the middle section (222 and 224, respectively) are cores and the first, third, and fifth regions (221, 223, and 225, respectively) are cladding under light transmitting conditions. The second and fourth middle regions are also spaced sufficiently closely that the second and fourth regions evanescently couple when light is transmitted into at least one of the second and fourth regions. The second middle region 222 is adjacent to the second lower region 212, the second upper region 232, and the first and third middle regions (221 and 223, respectively), and the fourth middle region 224 is adjacent to the fourth lower region 214, the fourth upper region 234, and the third and fifth middle regions (223 and 225, respectively). At least one of the regions is a hybrid region formed of a passive or electro-optic material such that at least one of the following conditions is satisfied:
1. in a cross-section taken at one point along the path of the optical signal, at least one of the second or fourth lower or upper regions or the first, second, third, fourth, or fifth middle regions has a hybrid portion, and in a cross-section taken at another point along the path of the optical signal, the same region has a non-hybrid portion;
in a cross-section taken at one point along the path of the optical signal for evanescently-coupled cores:
2. when the second or fourth lower region is the hybrid region, the other of the second or fourth lower region is formed of a cladding material having a refractive index that differs from the refractive index of the hybrid region;
3. when the second or fourth upper region is the hybrid region, the other of the second or fourth upper region is formed of a cladding material having a refractive index that differs from the refractive index of the hybrid region;
4. when the first, third, or fifth middle region is the hybrid region, at least one of the other of the first, third, or fifth middle region is formed of a cladding material having a refractive index that differs from the refractive index of the hybrid region; and
5. when the second or fourth middle region is the hybrid region, the other of the second or fourth middle region is formed of a core material having a refractive index that differs from the refractive index of the hybrid region.
Further, the invention provides new methods of making these structures. The methods place a material of different optical properties (e.g. a different refractive index) either (1) within a core in the structure or (2) within the cladding of the structure and sufficiently close to a core to affect the electric field of an optical signal being carried by the core. A rib-based method can be used to make a structure of this invention, wherein a rib of core material is formed as the structure is made, and either a portion of the rib or a portion of the cladding abutting the rib is a hybrid portion. One rib-based method is based on forming a cavity in a layer of a first core material, filling the cavity with a second core material, and removing a sufficient amount of the first core material to form a core having a length, a width, and a height such that the core has a portion along its length wherein the second core material spans the width and height of the core, and the second core material is located between two portions of the first core material of the core. Another rib-based method is based on forming a core comprised of a core material on a layer of a first cladding material and placing a second cladding material adjacent to at least one side of the core such that the second cladding material abuts that side of the core.
Another method for making a structure of this invention is a trench-based method, wherein a channel is formed and the channel is filled with core material as the structure is made, and either a portion of the core or a portion of the cladding abutting the core is a hybrid portion. One trench-based method is based on forming a channel in a layer of a cladding material, filling at least a portion of that channel with a first core material, forming a void in the first core material, and filling the void with a second core material. Another trench-based method is based on embedding a region of a first cladding material into a layer of a second cladding material, and forming a core within the layer of cladding such that both cladding materials abut the core on the same side of the core.
A temporary filler may be used in the processes described above. The temporary filler is placed in at least a portion of the structure (the core and/or the cladding) during manufacturing to allow portions of the structure to be fabricated of a material that differs from its surrounding material. The temporary filler is masked and partially etched, a first material is placed into the vacancies created by removing some of the temporary filler, and the remainder of the temporary filler is subsequently removed and replaced with a material that differs from the first material. This method can be used in the trench-based manufacturing process, wherein cores are formed in trenches cut into a substrate, or a rib-based manufacturing process, wherein rib cores are formed by etching a substrate and subsequently filling-in the etched portion with a cladding material. These methods produce regions of cores and/or cladding that have e.g. different refractive indices from surrounding materials.
Among other factors, the invention is based on the technical finding that a hybrid waveguide structure made by etching a substrate and using a temporary filler to provide cores or cladding with different refractive indices provides: (1) isolation between cores that can be varied; (2) a very small turning radius for cores; (3) very consistent performance between one batch of waveguides and/or switches and subsequent batches of waveguides and/or switches; (4) accurately-controlled dimensions and consistent performance because of the method of making the structure; (5) little overall loss of optical signal despite the use of materials in the structure that create high signal losses; (6) smaller devices or devices that have more components for a given size; and (7) unique device structures that act as filters, tapers, and switches that could not be made using a single material set. Further, the methods supplied by this invention align major structural elements such as hybrid cores to very accurate dimensions because these elements are established in a single photolithographic step. Also, the methods of this invention require few photolithography steps in which a substrate must be removed and repositioned within a stepper multiple times, so that cores and cladding can be made to precise dimensions. These technical findings and advantages and others are apparent from the discussion herein.